This invention relates to an improved method of producing thermosetting emulsion polymer mixtures containing glycoluril or melamine crosslinker where the polymeric mixture is heat treated to produce a minor amount of heat reaction between the glycoluril or melamine and the coreactive emulsion polymer. The reactive emulsion polymer can contain reactive hydroxyl, carboxyl, or acrylamide groups adapted to be crosslinked by the glycoluril or melamine to produce a cured coating film as set forth in commonly assigned U.S. Pat. No. 4,442,257 and Ser. No. 413,728 filed Sept. 1, 1982, now U.S. Pat. No. 4,444,941, and the same are incorporated herein by reference.
Glycoluril compositions are known for use in solvent based coating compositions in U.S. Pat. No. 4,064,191 and for use in powder coatings in U.S. Pat. No. 4,118,437. Aqueous glycoluril coatings are disclosed in prior patent U.S. Pat. No. 4,442,257 and said Ser. No. 413,728.
It now has been found that substantially improved clear and pigmented coatings based on glycoluril or melamine and reactive emulsion polymers can be produced by heat reacting the mixture for a limited period of time prior to use to provide a minor amount of reaction between the glycoluril or melamine and the reactive emulsion polymer. Heat treating the aqueous polymer mixture at temperatures above about 50.degree. C. for times of about one hour or more surprisingly provides substantially improved cured films exhibiting improved gloss, clarity, coating holdout over porous substrates, improved blocking resistance and a desirable rate of cure as well as enabling the use of considerably reduced amounts of glycoluril or melamine crosslinker to obtain excellent cured crosslinked film. The heat treating step of the aqueous polymeric mixture is believed to provide a minor amount of coreaction between the glycoluril or melamine and the reactive emulsion polymer whereby the glycoluril or melamine is believed to be physically drawn into or incorporated into the reactive emulsion polymer molecule. It is theorized that minor coreaction between the glycoluril or melamine and the reactive groups on the reactive emulsion polymer cause the glycoluril to diffuse into or associate with the reactive emulsion polymer and additionally provide an interpenetrating polymeric network and close association between the two polymer structures. These and other advantages of this invention will become more apparent by referring to the detailed description and the illustrative examples.